1. Field of the Invention
The present invention relates to an FM detector circuit using a phase-shifting circuit which can be readily constructed in the form of an integrated semiconductor circuit and requires no adjustment.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings is a circuit diagram showing an example of FM detector circuit comprising a conventional quadrature detector circuit, which comprises a phase-shifting circuit 20 for causing the phase of a limiter signal (IF signal) derived from a limiter circuit to be shifted by 90 degrees at the center frequency thereof, and a multiplier circuit 21 adapted for phase-detecting the limiter signal. Limiter signal derived from the limiter circuit is applied directly to one of the input terminals of the multiplier circuit 21, and a signal derived from the phase-shifting circuit 20 is passed to the other input terminal of the multiplier circuit 21; thus, the limiter signal is phase-detected in the multiplier circuit 21 so that FM detection output is provided.
The quadrature detector circuit is an FM detector circuit which is relatively well adapted to be constructed in the form of an integrated semiconductor circuit. However, the multiplier circuit 21 is constructed in the form of an integrated semiconductor circuit, but the phase-shifting circuit 20 comprises a capacitor, coil and so forth, i.e., it is an external component which is mounted onto a printed circuit board.
Further, adjustment operation for adjusting the phase-shift characteristic of the phase-shifting circuit 20 is effected by adjusting the resonance frequency of the phase-shifting circuit 20 by changing the inductance value of the coil by moving the magnetic core thereof for each tuning frequency.
In the FM detector circuit of FIG. 1, the multiplying circuit 21 is constructed in the form of an integrated semiconductor circuit, but the phase-shifting circuit 20 is an external component which is mounted onto a printed circuit board, as mentioned above. Hence, the FM detector circuit shown in FIG. 1 is disadvantageous in that it requires an increased number of parts and thus the manufacturing cost is increased. Another disadvantage is that it requires such troublesome adjustment operation that adjustment is effected for each tuning frequency as mentioned above.